feat: add concurrent scope isolation

[Lojhan]

Summary

• add a dedicated concurrent isolation test suite for react
• validate both paths:
  • contamination behavior when scope hooks are unavailable
  • isolation behavior when scope hooks are available
• keep react runtime implementation lean while making isolation behavior explicit in tests

Validation

• npm run lint
• npm test

Notes

• branch was created from updated main and synced before development
• this PR focuses on test coverage and isolation diagnostics